1. Field of the Invention
The invention relates to techniques for detecting and canceling intermodulation distortion in amplifiers, including radio frequency amplifiers.
2. Description of the Related Art
The present invention relates to the reduction of distortion in electrical circuitry. More particularly, the present invention relates to distortion reduction circuitry particularly suitable for amplifiers.
Radio frequency (RF) systems, such as cellular and microwave communication systems, typically require input signals to be fed into an RF power amplifier. Unfortunately, all amplifiers tend to distort the input signal at some power level and to generally cause the amplifier output to contain undesirable distortion products, spurious products, noise perturbations and intermodulation (IM) products. For purposes of simplicity, these will collectively be referred to as IM products. These IM products can cause undesirable interference over the amplifier frequency range. It is for these reasons that most amplifiers must incorporate some form of correction mechanism into the signal flow path through the amplifier.
One conventional method for reducing IM products uses a spectral analysis approach. This approach involves scanning the output of the amplifier using a receiver tuned to the frequency of candidate IM products. The IM level is measured and a linearizer is manually adjusted to minimize the IM product. This procedure is repeated until each of the IM products have a magnitude below a predetermined acceptable level. In a similar approach, as described in U.S. Pat. No. 4,580,105 issued to Myer, a portion of the output signal is taken and combined with an input signal which has been adjusted in phase and amplitude. The signal combination isolates the distortion component, which is then adjusted in phase and gain. This isolation distortion component is then added back to the output signal by means of a coupler to eliminate the distortion component. Unfortunately, the amount of distortion reduction available using this feed forward technique is limited by the accuracy of the gain and phase adjustments.
In addition, there appears to be a problem in the number of samples realized in several basic feed forward techniques. Samples in these systems are available only at the time that the phase detector circuit detects a correlated signal. This leads to poor error signal resolution and it is this error signal that is used to cancel corresponding spurs.
Moreover, it is believed that many cellular communication base stations do not function at full capacity because of serious linearity problems in the base station""s components. In addition, the need for increased capacity accompanying the expansion of cellular communications is forcing requirements for transmission standards of RF equipment to become too stringent to be met by construction of most present day amplifier circuits. Moreover, both Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) modulation require greater linearity and an absence of IM products that can not be routinely obtained by presently available high efficiency RF power amplifiers. Accordingly, it is clear that present correction techniques for eliminating IM products are not acceptable for evolving communications systems.
Thus, there is a need for a cancellation circuit that reduces IM products in electronic circuitry such as power amplifiers.
It would also be highly advantageous if the IM cancellation circuit were inexpensive to manufacture and highly reliable.
Moreover, it would be highly advantageous if the IM cancellation could be provided in a compact and lightweight constriction.
Briefly, in accordance with the invention, I provide an improved apparatus and method for automatically reducing IM products in electrical circuits. It is believed that this IM cancellation circuit is particularly suitable for application with amplifiers that amplify an input carrier signal.
In its most basic form, the IM product cancellation circuit includes a first coupler for sampling an input carrier signal. The coupler isolates a portion of the signal, preferably 6 dB below the input carrier signal so as to not substantially affect the energy level of the input carrier signal. This sample signal is then routed to a stepping spectrum analyzer which detects and measures the frequency and amplitude of the signal components of the carrier signal including its intended components and any IM products in the carrier signal. The stepping spectrum analyzer transmits the information relating to the frequency and amplitude of the different signal components in the carrier signal to a processor. Meanwhile, the processor stores information relating to the identity of the amplitude and/or frequency or frequency bands of intended signal components in the carrier signal. This information is typically in the form of look-up tables which identifies the amplitude or frequency, or in the case of TDMA or CDMA systems, the frequency band where the intended signal components are expected to reside. In this manner, the processor, by referencing this look-up table, can determine whether the signal components detected and measured by the stepping spectrum analyzer are intended signal components or unintended IM products.
The IM product canceling circuit further includes a variable voltage controlled oscillator, variable amplifier and variable phase shifter. The voltage controlled oscillator, amplifier and phase shifter are connected in series for creating an oscillator signal which can be controlled in amplitude and phase. The operation of the voltage controlled oscillator, amplifier and phase shifter are all controlled by the processor. Upon recognition and measurement of an IM product by the stepping spectrum analyzer and processor, the processor controls the voltage controlled oscillator, amplifier and phase shifter to produce an oscillating signal which is substantially identical in amplitude and frequency to a detected IM product but shifted in phase by 180 degree. This oscillating signal acts as an IM cancellation signal as the signal is fed back into the original carrier signal by means of a coupler or the like, such as a combiner. The original carrier signal is then output from the coupler retaining all of the original characteristics of the intended signal components except that the unintended IM product is canceled by the cancellation signal.
In a preferred embodiment, the IM cancellation circuit includes a feedback circuit. After the IM product is canceled, the output carrier signal is sampled by splitting the output signal with a coupler or the like to produce a sample of the output carrier signal. This sample of the output carrier signal is routed back to, in turn, the stepping spectrum analyzer and processor. The stepping spectrum analyzer and processor are thus able to confirm that the IM product has been canceled. In the alternative, where it has been determined that the cancellation signal is being transmitted at an incorrect frequency, amplitude or phase, the voltage controlled oscillator, amplifier and phase shifter are automatically adjusted by the processor until the cancellation signal correctly cancels the unintended IM product.
As would be understood by those skilled in the art, the aforementioned described cancellation circuit would only cancel a single unintended IM product. Where the carrier signal is likely to have more than one unintended IM product, the cancellation circuit is provided with a plurality of variable voltage controlled oscillators, variable amplifiers and variable phase shifters. Each set of these voltage controlled oscillators, amplifiers and phase shifters are connected in series to the processor for creating a plurality of cancellation signals where necessary.
In an additional preferred embodiment, the IM product cancellation signal is adapted for use with an amplifier. A portion of the output from the amplifier is forwarded to the stepping spectrum analyzer for recognition and measurement of not only the intended signal components but also any IM products. This information is sent to the processor which determines whether the signals received by the stepping spectrum analyzer are intended signal components or unintended IM products. Whether the carrier signal includes IM products can be determined by comparison of the received signals to look-up tables, which identify the amplitude and/or frequency band of the intended signal components. If a detected signal is not listed in the look-up table, the signal is recognized as an IM product.
In an additional embodiment, the amplifier input signal is split with a first component going to the amplifier and a second component, typically 6 dB below the signal input to the amplifier, being sent to the stepping spectrum analyzer for analysis. The frequencies of signals originally input in the amplifier are identified as intended signal components and stored in the processor. The frequencies of the intended signal components are then compared with the frequencies of signals output from the amplifier. IM products can then be identified as appearing at frequencies not appearing in the amplifier input signal. The processor then controls the variable voltage controlled oscillator and variable amplifier to produce a cancellation signal having a frequency and amplitude substantially equal to the frequency and amplitude of the IM product. The cancellation signal is then shifted in phase 180 degree with respect to the phase of the IM product and combined with the carrier signal by means of a coupler or the like. The resulting carrier signal is thus xe2x80x9ccleansedxe2x80x9d with the IM product removed.
In still an additional preferred embodiment, the amplifier circuit includes a traditional feed forward correction circuit. The feed forward correction circuit will typically cancel much of the noise and IM products of an amplifier. However, the components of the feed forward correction circuit, such as combiners, amplifiers and phase shifters, etc., typically add additional IM products to the amplifier output. These IM products can be canceled by application of the IM cancellation circuit of the present invention. As described above, the output signal from the amplifier is split with a component going to the stepping spectrum analyzer. IM products are then recognized by the processor by use of a look-up table or by comparison to an analysis of the original amplifier input signal. The processor then controls the variable controlled oscillator, variable amplifier and variable phase shifter to produce a cancellation signal substantially equal in frequency and amplitude to the IM product but shifted in phase by 180 degree. This cancellation signal is then combined with the amplifier output signal to cancel the unintended IM product.
A preferred embodiment of the stepping spectrum analyzer for use in application with the present invention is substantially similar to the automated frequency stepping noise measurement test system disclosed and described in copending U.S. patent application Ser. No. 09/313,435 which is incorporated by reference herein. In a preferred embodiment, the stepping spectrum analyzer includes a variable low noise source for producing a low noise signal having an adjustable frequency. The variable low noise source includes two outputs for outputting identical low noise signals, or is coupled to a splitter for splitting a low noise signal into two identical low noise signals. The first low noise signal is routed to a coupler that combines the first low noise signal with that portion of the input carrier signal comprising the sample signal. The second low noise signal is routed to a variable phase shifter, which adjusts the phase of the second low noise signal to be 90 degrees out of phase (in phase quadrature) with respect to the first low noise signal which has been combined with the sample signal. The sample signal, carrying the intended carrier signal components and any unintended IM products, and the first low noise signal are then sent together to a mixer where the signal is mixed with the second low noise signal which has been adjusted in phase. Due to the inherent characteristics of a mixer, the low noise source signals are canceled in the mixer output signal by having the second low noise signal shifted in phase 90 degrees with respect to the first low noise signal. Moreover, the mixer is nonlinear producing an output signal including only a single sideband with respect to the frequency of the low noise signal but having double the amplitude. The signal output from the mixer, hereinafter referred to as a xe2x80x9cmeasurement test signalxe2x80x9d, is then sent to a variable low noise matching amplifier. The variable low-noise matching variable-gain amplifier both amplifies the measurement test signal and acts as a buffer. The matching variable-gain amplifier is constructed to add very low noise or IM products so as to not interfere with the IM measurements by the stepping spectrum analyzer and provides for amplification of the measurement test signal to enhance the ability of the stepping spectrum analyzer to measure any IM products in the original sample signal.
After passing through the low-noise matching amplifier, the measurement test signal is sent to an analog-to-digital converter (ADC) which converts the analog measurement test signal into digital data. The digital data is then transmitted to a processor for evaluation. The processor of the stepping spectrum analyzer may be separate and independent from the processor which controls the variable voltage controlled oscillator, variable amplifier and phase shifter which creates the IM product cancellation signal, or both processor functions may be combined in a single processor. The processor of the stepping spectrum analyzer uses standard, windowed, fast or discreet Fourier transforms to accurately measure the characteristics of the measurement test signal and whether it includes any IM products. These Fourier transforms are known to those skilled in the art and will not be discussed in detail herein.
The processor of the stepping spectrum analyzer is connected to a plurality of control lines to the variable amplifier, variable low noise source, variable phase shifter and variable low noise matching amplifier. These control connections enable the processor to automatically set levels and make adjustments to the amplifier, low noise source, phase shifter and matching amplifier to xe2x80x9cstepxe2x80x9d the low noise source across a desired frequency band at different offset frequencies in order to identify the IM products in that frequency band. To control the stepping spectrum analyzer, the processor takes the digitized output from the ADC to both calibrate the system and to ensure that the amplifier, low noise source and phase shifter are set to correct levels. More particularly, the output from the ADC enables the processor to determine whether the low noise source is providing the first and second low noise signals at a correct frequency.
By evaluating the output from the ADC, the processor can also confirm that the phase shifters are properly maintaining the signals received by the mixer in phase quadrature. If any of these components are not functioning optimally, the processor automatically makes required adjustments to ensure proper recognition and measurement of any IM products in the carrier signal. Once the frequency characteristics are recognized and measured by the stepping spectrum analyzer, this signal information is sent to the processor which controls the variable voltage controlled oscillator, variable amplifier and variable phase shifter which produce the cancellation signal. This processor (by comparison of the signal information to internal look-up tables) determines whether the carrier signal is carrying any unintended IM products and controls the variable controlled oscillator, variable amplifier and variable phase shifter to produce a cancellation signal which is combined with the carrier signal to cancel an unintended IM product.
In one embodiment, phase shifters are provided in relatively lower-power circuit paths rather than relatively higher-power circuit paths in order to allow relatively lower-power phase shifters to be used.
It is thus an object of the present invention to provide an improved apparatus and method for canceling IM products in electronic circuitry.
It is an additional object of the present invention to provide an IM cancellation circuit which is inexpensive to manufacture, highly reliable, compact and of lightweight construction.
These and other further advantages of the present invention will be appreciated by those skilled in the art upon reading the following detailed description with reference to the attached drawings.